Tuesday, May 15, 2012

The Beauty of Big

One rather obvious trait of pterosaurs, compared to other flying animals, is that they had a tendency to get rather large. There has been much to do about how they could get so large (as most readers of this blog know, I and quite a few others now prefer the explanation that pterosaurs were quadrupedal launchers). However, it's a bit trickier to tackle the problem of why some of them became so large.

What might select for giant size in pterosaurs? It's probably not something that can be answered definitively, but there are some plausible options. One of them relates to the issue of long-range travel. I talked a bit about this potential advantage here. The gist is this: being large makes long-distance travel more feasible for most animals, particularly flyers. Some reasons this happens include:

1) Large flyers can carry more fuel.

2) Large flyers travel at higher speeds, on average.

3) Large flyers are less affected by adverse weather conditions.

4) Large flyers use less fuel per unit body mass, per unit time.

This means that, on average, a big flying animal can go longer between stops on long migrations, and gets to their destination more quickly, than a small flyer. This might be particularly important for a flyer which feeds on resources that are patchy in their distribution. This, in turn, might suggest (very tentatively) that animals like azhdarchids had a tendency to travel long distances between food sources.

Of course, there are a host of other advantages to being large, as well: big animals are harder for predators to kill, can eat larger prey, and can (in some cases) better defend young. For egg-laying animals, large size often improves fecundity. So there is no way to say for certain exactly why some pterosaurs grew quite large, but it does raise questions of general biological interest.

Hey Alex. Great question! It was once thought that pterosaur wings were quite fragile, but specimens with soft tissue preservation indicate that the wing membranes were actually rather highly reinforced (as soft tissues go), so damage to the membrane was probably not of much concern in heavy winds.

Colin Palmer suggested more recently that strong winds might be a problem for pterosaurs based on his work that suggested they flew at rather high lift coefficients. However, those experiments looked mostly at membranes that passively cambered with little control within the membrane. In reality, at higher speeds (or in high winds) pterosaur membranes could probably tighten quite a bit to resist passive cambering. Colin's work was critical, because it gives us the upper limit of performance for low speed flight (which we need to estimate takeoff, landing, turning, etc).

As it turns out, pterosaurs probably did very well flying slowly, which is pretty sweet. Colin's work suggests that some large pterosaurs may have been loitering adapted, rather than specialized to high speed soaring (also cool!). But I would expect many large pterosaurs, particular marine species, to manage well at higher speeds and winds, too (particularly things like Pteranodon).